A loudspeaker is a device which converts an electrical signal into an acoustical signal (i.e., sound) and directs the acoustical signal to one or more listeners. In general, a loudspeaker includes an electromagnetic transducer which receives and transforms the electrical signal into a mechanical vibration. The mechanical vibrations produce localized variations in pressure about the ambient atmospheric pressure; the pressure variations propagate within the atmospheric medium to form the acoustical signal. A horn-type loudspeaker typically includes a transducer assembly, an acoustical transformer, and an acoustical waveguide or horn.
FIG. 1A is a sectional view of a transducer assembly 10, an acoustical transformer (alternately known as a phase plug) 11, and a horn 12, as disclosed in U.S. Pat. No. 6,094,495, which is incorporated by reference herein. The transducer assembly 10, shown in more detail in the sectional view of FIG. 1B, includes a cone-type driver including a voice coil 13, an annular cone having an outer portion 14 and an inner portion 16, and a dust cap 17 attached to and covering the voice coil 13. The sectional view of FIG. 1B shows one half of the transducer assembly 10 sectioned at the central axis CA, which is preferably the axis of propagation of the acoustic energy generated by the loudspeaker system. Both the outer portion 14 and the inner portion 16 of the cone are in the form of a cone truncated at both ends. The periphery of the smaller end of the outer portion 14 and the periphery of the larger end of the inner portion 16 coincide at a junction 18, and the cone is fixedly attached to the voice coil 13 at the junction 18. The dust cap 17 is fixedly attached to the inner portion 16 of the cone, and intersects the central axis CA at the dust cap peak 19.
The distance from the junction 18 to the dust cap peak 19 along the inner portion 16 of the cone and the dust cap 17 is designated as D1. The distance from the junction 18 to an outer periphery 15 along the outer portion 14 of the cone is designated as D2. Preferably, the distance D1 is substantially equal to the distance D2. Mechanical vibrations travel through the outer portion 14 and the inner portion 16 along equidistant paths D2 and D1, respectively, and thus the dust cap peak 19 and the outer periphery 15 of the outer portion 14 of the cone produce acoustical signals which have a substantially equal time relationship.
Still referring to FIG. 1B, the acoustical transformer 11 (i.e., the phase plug) is typically disposed adjacent to the transducer assembly 10 so as to reduce the volume of an air chamber 2 driven by the transducer assembly 10. This in turn reduces the mechanical reactance that only permits mechanical vibrations at lower frequencies, to thereby allow mechanical vibrations at higher frequencies also. As illustrated, the rear face of the phase plug 11 facing the transducer assembly 10 includes a first conical section 3 which faces the outer portion 14 of the cone, and a second conical section 4 which faces the inner portion 16 of the cone and the dust cap 17. The first and second conical sections 3 and 4 meet at a peak 5.
FIG. 1C shows the rear face of the phase plug 11 facing the transducer assembly 10. A solid line, labeled 5, is used to indicate the location of the peak 5. The central axis CA is shown as a point at the center of the phase plug 11. As shown, the phase plug 11 includes a plurality of elongated radial slots 6, with six being shown as 6a through 6f, extending radially from an inner radial location “R” out to the edge 7. Each of the elongated slots 6a-6f forms with the phase plug 11 an internal acoustical waveguide which extends in the direction of the central axis CA. These various waveguide paths through the slotted phase plug 11 preferably provide the same effective length through which acoustical signals from the diaphragm (the cone in this case) travel so that the signals produced from the front face of the slotted phase plug 11 have a substantially equal time relationship.
The acoustical waveguide or a horn 12 receives the acoustical signal radiated by the transducer assembly 10 and the phase plug 17 and directs the signal in a particular direction.
In the loudspeaker as described above in reference to FIGS. 1A-1C, the slots 6a-6f serve to reduce the amount of path length variation to thereby achieve substantially coherent acoustical signal transmission and production. There remains, however, a range of transient variations among various signal paths through the slotted phase plug 11 (e.g., some signals from the diaphragm not entering the nearest slot, re-entering multiple slots, etc.). In fact, any type of phase plug, due to its particular configuration, inherently suffers from a certain degree of transient variations among various signal paths through the phase plug. Although the time difference may be only a fraction of a millisecond, it is enough to color the resulting acoustical signal radiated from the transducer assembly 10 such that the acoustical signal is not a true representation of the original acoustical source.
The present invention is directed to creating a series of digital signal processing (DSP) correction/preconditioning filters to be incorporated into a loudspeaker system, to correct sound inaccuracies caused by various physical behaviors of loudspeaker components, such as transient smear caused by the multiple paths through a compression driver phase plug as described above.